1. Field of the Invention
The invention relates to a hydraulic control system for a vehicle, the hydraulic control system including a mechanical oil pump that is driven by a driving force source for the vehicle, such as an engine and a motor, and an electric oil pump that is driven by an electric motor different from the driving force source.
2. Description of Related Art
Japanese Patent Application Publication No. 2013-142458 (JP 2013-142458 A) describes the invention regarding an oil supply system including a mechanical oil pump and an electric oil pump as pumps for supplying oil. The oil supply system described in JP 2013-142458 A includes an electromagnetic valve as changing means for changing a communication state between a first oil discharge passage and a second oil discharge passage. The mechanical oil pump discharges oil through the first oil discharge passage. The electric oil pump discharges oil through the second oil discharge passage.
Japanese Patent Application Publication No. 2011-978 (JP 2011-978 A) describes the invention regarding an auxiliary pump drive control system. A mechanical oil pump and an electric oil pump (auxiliary pump) are provided. The mechanical oil pump is driven by an engine. The electric oil pump is driven by a driving source different from the engine. The auxiliary pump drive control system is configured to, when driving of the electric oil pump is started, drive the electric oil pump in the reverse direction. JP 2011-978 A also describes the configuration that the above-described electric oil pump is installed on the outer portion of a transmission case.
As in the case of the system described in JP 2013-142458 A or the system described in JP 2011-978 A, in the configuration that a mechanical oil pump and an electric oil pump are provided, usually, the output of an electric motor that drives the electric oil pump is considerably smaller than the output of a driving force source that drives the mechanical oil pump. Therefore, for example, when the electric oil pump is started up in a state where the viscosity of oil is high at a low temperature, the output of the electric motor may be insufficient for load on the electric oil pump, with the result that may not be possible to appropriately start up the electric oil pump.
In the system described in JP 2013-142458 A, at a startup of the electric oil pump, the open/closed state of the electromagnetic valve is changed such that oil flows back from the mechanical oil pump-side first oil discharge passage to the electric oil pump side. Thus, low-temperature high-viscosity oil remaining inside the electric oil pump or the second oil discharge passage is returned to an oil pan by the pressure of oil that is flowed back from the mechanical oil pump side. As a result, the inside of the electric oil pump or the second oil discharge passage is filled with oil raised in temperature at the mechanical oil pump side, and the electric oil pump is allowed to be early driven. However, by using the above-described electromagnetic valve, a system that controls the operation of the electromagnetic valve is required. Electric power needs to be externally supplied in order to actuate the electromagnetic valve. In addition, the electromagnetic valve is generally expensive. Therefore, in the system described in JP 2013-142458 A, the operation of the electromagnetic valve needs to be controlled. The energy efficiency of the system decreases by the amount by which electric power is consumed at the time of activating the electromagnetic valve. In addition, the cost of the electromagnetic valve becomes a factor of an increase in the cost of the system.
As in the case of the system described in JP 2011-978 A, by driving the electric oil pump in the reverse direction when driving of the electric oil pump is started, it is possible to return oil, remaining inside the electric oil pump and having a high viscosity at a low temperature because of outside air, to the oil pan. Together with this, it is possible to introduce relatively high-temperature low-viscosity oil stored in the oil pan and distribute the oil to the inside of the electric oil pump. Therefore, in the system described in JP 2011-978 A, by driving the electric oil pump in the reverse direction as described above and then driving the electric oil pump in the forward direction, it is possible to drive the electric oil pump at low load. However, even with such a configuration, eventually, the electric motor that drives the electric oil pump is required to provide power for making it possible to drive the electric oil pump in the reverse direction even in a state where the viscosity of oil inside the oil pan is high. An increase in the power of the electric motor leads to an increase in the size and weight of the system and an increase in cost.
In addition, at a startup of the electric oil pump in a state where the viscosity of oil is high as described above, the startup of the electric oil pump may not be appropriately carried out. In such a case, it is difficult to accurately determine whether it is due to insufficient power of the electric motor that drives the electric oil pump or it is due to a failure of the system. When the above-described failed startup of the electric oil pump is due to insufficient power of the electric motor, it is possible to deal with such a failed startup by retrying a startup of the electric oil pump or driving the mechanical oil pump by starting up the engine. On the other hand, when the above-described failed startup of the electric oil pump is due to a failure of the system, it may be necessary to quickly stop or suppress the driving of the electric oil pump in order to suppress useless consumption of electric power for driving the electric motor or prevent a further secondary failure.